1. Field of the Invention
The present invention relates to pulse-echo ranging system and, more particularly, to a method for echo processing in a pulse-echo ranging system, where the method comprises transmitting an energy pulse to a target, receiving echo pulses and converting them into an analog echo signal, and processing the analog echo signal to identify an echo from the target and determining the distance from the propagation time of the identified echo, where the advanced stage of the processing is performed digitally.
2. Description of the Related Art
In level measurement applications, pulse-echo ranging systems, which are also known as time-of-flight ranging systems, are commonly used for determining the distance to a target object (e.g., reflective surface of a material in a container) by measuring how long after transmission of an energy pulse the reflected pulse or echo is received. Such devices typically use ultrasonic pulses or pulsed radar or microwave signals.
In general, pulse-echo ranging systems include a transmitter for repeatedly transmitting the energy pulses and a receiver for receiving the reflected energy pulses or echoes. The transmitter and receiver may be combined in a single unit. The receiver provides an analog echo signal that contains any received echo, whether the received echo is from the target, or from clutter or noise. The echo signal may be amplified and/or filtered before it is digitized and stored as an echo profile. A signal processor identifies echoes of interest in the stored echo profile and calculates the distance or range of the target based on the transmit times of the transmitted energy pulses and the identified echo pulses.
A commonly used technique for finding echoes in an echo profile involves generating a time varying threshold (TVT) curve providing a line on the echo profile that is above the noise level in the echo profile. Valid echoes appear above the TVT curve. For identifying the echo of interest and determining its temporal position on a temporal axis, a variety of known techniques may be used, such as correlation or determining the echo's leading edge, trailing edge, peak or center of mass.
It is known from, for example, U.S. Pat. No. 5,436,580 to determine the beginning of an echo pulse within a received signal by developing a time dependent first derivative of the signal and comparing the signal with the derivative and with a reference threshold. Here, the leading edge of the echo pulse is indicated when the received signal exceeds both its derivative and the reference threshold.
In pulse-echo ranging systems, i.e., those having embedded microprocessors or microcomputers, the memory for storing the echo profile is limited, which limits the number of samples or their size (i.e., resolution) that can be stored. The worst case occurs when the measurement range is set to its maximum which means that the samples are furthest apart because the full range must now fit into the fixed size memory.